since yesterday no visible content in widgets (only text, html)

You can try to remove all widgets and then save to see if that fixes it.

Also, you can turn on debug mode to see if there’s a fatal error while saving the widgets:
https://developer.wordpress.org/advanced-administration/debug/debug-wordpress/

Insert this BEFORE /* That’s all, stop editing! Happy blogging. */ in the wp-config.php file

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );

Then, try to save the widgets and see if there’s a PHP error:
wp-content / debug.log

Later, you can turn off debug with false value and remove this log file.